Measurement of transpiration constitutes an essential element in the understanding of the physiology of trees and of the dynamics of water transfer in forest plantations. Within the framework of his research in this particular area, the inventor found himself confronted with the problem of the continuous measurement of the flow of raw sap in the trunks of trees. There currently exist numerous methods for measurement of transpiration flow. Among the best known can be cited the so-called "heat-impulse" method described notably by Swanson R. H. in "An instrument for detecting sap movement in woody plants", Sta. Pap. Rocky Mt. For. Range Exp. Sta. No. 68, 1962, and "Velocity distribution patterns in ascending xylem sap during transpiration" in Flow, its Measurement and Control in Science and Industry, Eds. Rodger and Dowdell, Instrument Society of America, Vol. 1, 1425-30, 1974.
However, none of these methods present the specifications required for allowing low-cost monitoring of transpiration in trees in forest plantations. The heat impulse method in particular has two drawbacks: its punctual character in the trunk and its imprecision under conditions of weak transpiration.